Known piezoelectric polymers include a polypeptide type, such as poly(.gamma.-benzyl L-glutamate); an electret type, such as polyvinyl chloride; and a ferroelectric substance type, such as polyvinylidene fluoride, a vinylidene fluoridetrifluoroethylene copolymer, and a vinylidene cyanide-vinyl acetate copolymer. The most typical among them is a film of polyvinylidene fluoride, which has already been put to practical use in ultrasonic transducers (see JP-A-55-151896 and JP-A-56-112200 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
Polyvinylidene fluoride, a vinylidene fluoridetrifluoroethylene copolymer, and a vinylidene cyanide-vinyl acetate copolymer, which are synthetic polar piezoelectric polymers of the ferroelectric substance type, are oriented by drawing and the application of an electrical field. The mechanism of retention is spontaneous polarization or freeze-polarization, and the orientation state is uniaxial polar orientation. A drawing treatment and a polarizing treatment are required before these materials exhibit piezoelectric properties. In particular, polyvinylidene fluoride has the highest piezoelectric properties. However, it has a dielectric constant of 13, which is relatively high as compared with other polymeric materials, so that the piezo g-constant (open circuit voltage per unit stress) obtained by dividing a piezo d-constant by a dielectric constant is not so great. Therefore, it has somewhat poor sound-to-electricity transformation efficiency, although it has satisfactory electricity-to-sound transformation efficiency.
Films of electret type polymers, such as polyvinyl chloride, are endowed with piezoelectric properties by orientation of the polar groups by polarizing treatment. Piezoelectricity of these materials is not as high as the polymers of the ferroelectric substance type.
To the contrary, artificial oriented films of naturally-occurring polymer-related substances, such as poly(.gamma.-benzyl L-glutamate) which are of the polypeptide type, DNA, and polyhydroxybutyrate, have their orientation controlled by dynamic drawing. The mechanism of retention is a crystal structure, and the state of orientation is uniaxial orientation with no polarity. These films exhibit piezoelectric properties without being subjected to polarizing treatment in such a manner that polarization takes place in the z direction upon application of shear in the xy directions. However, since they have a molecular structure in which a main chain helix is surrounded by a long side chain, their piezoelectric properties are of the relaxation type and are not so strong. Further, since the mechanical strength of these materials themselves is insufficient, it is difficult to obtain rigid piezoelectric materials of irregular shape.
With respect to the artificial oriented film of polyhydroxybutyrate, there is a research report that piezoelectricity of the film wound around a fractured part of a bone is advantageous of for acceleration of the initial ossification (Shigeru Harada, et al., The 12th Research Meeting on Bone and Electrical Stimulation, No. 8 (1985)). However, the fact that the film has small piezoelectric constants d'.sub.14 of about 1.3 (pC/N) and e'.sub.14 of 3.5 to 4.0 (mC/m.sup.2) at room temperature and 10 Hz leaves doubt as to whether the piezoelectricity of the film makes a true contribution to the ossification in the initial stage. Even if it does, it seems that a remarkable effect on the acceleration of ossification would hardly be produced. Further, the artificial oriented film of polyhydroxybutyrate so slowly degrades in a living body that it remains unabsorbed for a long time even after a fracture completely heals. Besides, the safety of polyhydroxybutyrate in a living body has not yet been confirmed.